Downdraft gasifiers are well known and have been used for over 100 years. In the arrangement the biomass and oxidant both flow in a downward direction. The use of a downdraft gasifier results in a gas which is very low in tar concentration as the syngas passes through a char zone towards the lower section of the bed where significant tar destruction occurs. As the produced syngas requires minimal further clean up this type of gasifier has been found useful as an onboard gasifier for vehicle use during times of fuel shortages.
The downdraft gasifier also has a number of disadvantages. As the bed is supported on a grate it is possible for the biomass to plug the grate or bed, resulting in a non-even distribution (a maldistribution) of air through the bed, excessive pressure drop across the depth of the bed, and even the need to shut down the gasifier to clear the grate and the bed.
The biomass may also form bridges or channels, thereby forming low pressure drop “short-cuts” for the oxidant, which result in lower bed combustion, weak gas production and possibly increased rates of tar production.
Another problem is that the flame front can be difficult to stabilize. Depending on operating conditions, the flaming pyrolysis front may migrate to the top of the bed, resulting in unstable operation and/or upper combustion, again resulting in the need to shut down the system. One downdraft gasifier, namely, the Imbert design, overcomes this last problem through radial injection of oxidant only towards the lower bed. The flame front is thus naturally stabilized there—it cannot travel upwards due to a lack of oxidant above the point of injection. However, this technique lacks the ability to be scaled to higher throughputs due to a limitation in how far into the bed the radially directed jets can cause oxidant penetration. In effect, the upper sizing is dictated by how far the oxidant can penetrate into the bed.